Wireless or wired communications systems often rely on forward-error-correction (FEC) in order to control errors when information is transmitted over noisy communications channels. In such systems, the sender encodes the information to be transmitted using error correcting codes. Exemplary error correcting codes include block codes (i.e., ones that operate on fixed-size packets), convolutional codes (i.e., ones that may operate on streams of arbitrary length), or concatenated codes (i.e., ones that combine block codes and convolutional codes). Certain block codes can be represented by parity check matrices, such as high, medium and low density parity check (H/M/LDPC) codes. Reed-Solomon (RS) codes are an example of well-known block codes as they are not only employed in many wireless or wired communication systems such as broadcast systems (including HD Radio systems which are discussed further below), but also in consumer electronics and data storage systems such as disc drives, CDs and DVDs.
While many methods exist for decoding of LDPC codes such as ones based on belief propagation (BP) algorithms, such methods typically do not yield good performance when used to decode codes having higher parity check matrix densities, including MDPC and HDPC codes, such as RS or BCH codes. Thus, there is a need for improved systems and methods for decoding block codes (or concatenated codes that include block codes), particularly H/M/LDPC codes or any codes that could be represented by parity check matrices including RS codes, in a manner that improves the performance while keeping computational complexity reasonable.
Proper FEC decoding in communications systems also relies on the ability to determine as best as practically possible certain attributes of the communication channel. For instance, the channel response as well as noise power estimation, which together may be referred to as channel state information (CSI), are often estimated and used not only for error correction, but also for other processing functions such as coherent demodulation and diversity combining in order to achieve maximum-possible performance gains offered by those processing tasks. CSI estimation is also of importance in diversity combining systems.
To facilitate CSI estimation, pilot symbols are usually inserted in a stream of data symbols. Such pilot channel estimation relies on filtering techniques that have typically used filter lengths which either do not optimally account for noise effects or channel dynamics (i.e., the rapidity of channel variation). Thus, although prior art filter structures may be suitable for certain scenarios, they are not optimal when the system needs to operate over a wide range of channel dynamics, thereby adversely affecting performance. Accordingly, there is also a need to improve channel estimation techniques in additive white Gaussian noise (AWGN) and fading communication channels, which would result in improved decoding performance.
As discussed above, there is a need for improved techniques for decoding a wide variety of codes, including RS codes, which may be used in various systems including consumer electronics and data storage systems, as well as broadcast systems (where there is also a need to improve channel estimation) such as in HD Radio receivers. HD Radio refers to a digital radio technology that enables the transmission and reception of digital audio and data, addressing the limitations of aging analog broadcast transmission technology.
Current HD Radio systems are based on a particular type of multicarrier technology known as orthogonal frequency-division multiplexing (OFDM). A hybrid method of transmitting analog radio broadcast signals and digital radio signals simultaneously on the same frequency band is referred to as in-band on-channel (IBOC) transmission. IBOC transmission allows broadcasters to transmit both analog and digital signals on their existing assigned channels in the amplitude modulation (AM) or frequency modulation (FM) frequency range. On the other hand, all-digital HD Radio systems of the future (which are not yet deployed) are expected to only carry the digital HD Radio signal.
HD Radio systems typically transmit a system control data sequence for the purpose of system control data synchronization and, possibly, channel estimation. For example, the system control data sequence in FM HD Radio consists of synchronization bits, control bits, and parity bits, which are transmitted on pilot tones that are commonly referred to as the reference subcarriers. The differential phase-shift keying (DPSK) modulated pilot symbols are multiplexed onto OFDM symbol along with data symbols. The reference subcarriers on which pilot symbols are transmitted are distributed over the OFDM spectrum. Control and status information are collected to form system control data sequences and are transmitted on the reference subcarriers. Use of the system control data sequence for acquisition, tracking, channel estimation and coherent demodulation has been described in U.S. Pat. No. 6,549,544. Decoding of the system control data sequence is important for the system performance. The parity bits are inserted into the variable fields of the system control data sequence for error detection and prevention of error propagation at the end of each variable field due to differential encoding.
The DPSK modulated pilot symbols, in which the information is carried in the phase difference between adjacent bits, are decoded non-coherently at the receiver. Selected information bits in a system control data sequence may be repeated within the same system control data sequence and those repeated bits are transmitted on a set of reference subcarriers whose positions in the frequency band are known to the receiver such that frequency diversity could be exploited during the decoding process at the receiver.
In the current HD Radio receivers, all transmitted DPSK-modulated system control data sequences carried on the reference subcarriers are first non-coherently demodulated and then a majority voting is applied to those repeated bits to make a final bit decision of all repeated bits collectively. The final bit decision based on majority voting facilitates a correct decoding of those bits repeated in the system control data sequence, although some of the repeated bits in a system control data sequence may be corrupted when received. This process is commonly referred to as majority voting combining. In addition to the repetition of some bits in a system control data sequence, a small set of bits in a system control data sequence are protected by a parity bit, allowing detection of existence of bit errors in the set of parity-covered bits.
As for channel estimation, if the parity does not match, the parity field is considered unreliable and may not be used to estimate the channel response (or noise power). In this case, non-uniform interpolation could be applied.
In addition, existing HD Radio receivers rely on Viterbi decoders to decode convolutional codes in a manner that produces hard-decision decoded bits. For audio channels, these hard-decision outputs are passed to a conventional cyclic redundancy check (CRC) decoder for error detection, and then to a source audio decoder. For data channels, hard-decision outputs are passed to an algebraic RS decoder, also producing hard decision bits, followed by a conventional CRC decoder for error detection. Each operation is done once and in a sequential manner in the prior art. However, algebraic RS decoding on hard bit decisions out of the Viterbi decoder results in suboptimum performance and such an approach is not amenable to potential iterative decoding improvements.
From the foregoing discussion, it is clear that there is a need for improving the performance of decoders, and more generally, decoding block codes (or concatenated codes that include block codes), as well as channel estimation, in communication and other systems. Moreover, there particularly is a need for systems and methods that improve the decoding performance of AM and FM HD Radio receivers in order to extend the range/coverage of digital radio, preferably without modifying the existing HD Radio transmission or infrastructure.